Methods and apparatus for spatial separation of AC and DC electrical fields with application to fringe fields in quadrupole mass filters

ABSTRACT

Methods for spatially separating ac and dc electric fields in which materials are used having characteristics as dielectrics to the ac fields and as conductors to the dc fields. The apparatus for separating the fields utilizes homogeneous materials of moderate to high resistivity. The method is applied to the separation of fringe fields near the ends of quadrupole mass filters for the purpose of improving the efficiency of ejection and transmission of ions in mass filter devices.

United States Patent Fite [451 Feb. 18,1975

[ METHODS AND APPARATUS FOR SPATIAL SEPARATION OF AC AND DC ELECTRICALFIELDS WITH APPLICATION TO FRINGE FIELDS IN QUADRUPOLE MASS FILTERS [75]Inventor: Wade L. Fite, Pittsburgh, Pa.

[73] Assignee: Extranuclear Laboratories, Inc., Pittsburgh, Pa.

[22] Filed: Mar. 30, I973 [21] Appl. No.: 346,250

[52] US. Cl 250/292, 250/282, 250/290 [51] Int. Cl. IIOlj 39/34 [58]Field of Search 250/281, 282, 283, 290,

[56] References Cited UNITED STATES PATENTS Brubaker 250/292 3,617,73611/1971 Barnett et al. 250/292 Primary Examiner-Archie R. BorcheltAssistant Examiner-B. C. Anderson Attorney, Agent, or Firm-Mason, Mason& Albright [57] ABSTRACT Methods for spatially separating ac and dcelectric fields in which materials are used having characteristics asdielectrics to the ac fields and as conductors to the dc fields. Theapparatus for separating the fields utilizes homogeneous materials ofmoderate to high resistivity. The method is applied to the separation offringe fields near the ends of quadrupole mass filters for the purposeof improving the efficiency of ejection and transmission of ions in massfilter devices.

19 Claims, 7 Drawing Figures PATENTEB I 8875 3.867. 632

SHEET 1 OF 2 1 3i {MED FEB] 8 HTS SHEET 2 OF 2 1 METHODS AND APPARATUSFOR SPATIAL SEPARATION OF AC AND DC ELECTRICAL FIELDS WITH APPLICATION.TO FRINGE FIELDS IN QUADRUPOLE MASS FILTERS BACKGROUND TO THE INVENTIONThe quadrupole mass filter of W. Paul et al. described in U.S. Pat. No.2,939,952, issued June 7, 1960, consists of four substantially parallelhyperbolic sheet electrodes (or cylindrical rods), symmetricallydisposed about an axis. Opposite rods are electrically connected. On onepair of electrically connected oppositely disposed electrodes a dcvoltage, U, and an ac voltage of amplitude, V, are placed. On the otherpair of electrically connected oppositely disposed electrodes identicalvoltages, except having an electrical polarity opposite to the firstpair, are placed. With proper settings of the dc voltage and theamplitude of the ac voltages, ions of a given charge-to-mass ratio havestable trajectories and oscillate about the axis whereby they do notcollide with the electrodes; ions of other than the given charge-to-massratio are on unstable trajectories whereby they strike the electrodes.If ions are injected along the axis of the electrode structure, thosewith the given charge-to-mass ratio do not strike the electrodes andemerge from the opposite end of the electrode structure; however, ionswith other than the given charge-to-mass ratio are accelerated in thetransverse directions so that they collide with the electrodes andtherefore do not emerge from the opposite end of the electrodestructure. In this manner, the electrode structure functions as an ionmass filter.

As noted in U.S. Pat. No. 3,129,327 to W. M. Brubaker of Apr. 14, 1964,an ion entering the electrode structure must pass through fringe fieldsnear and beyond the end of the electrode structure. The ions must alsopass through a similar fringe field in emerging from the opposite end ofthe electrode structure. As pointed out in the aforesaid patent of W. M.Brubaker, the ratio of the dc field strengths to ac field strength inthe fringe fields are the same as in the electrode structure itself.Also, as disclosed in the aforesaid patent, an ion of the givencharge-to-mass ratio, which is stable within the electrode structureproper, is on an unstable trajectory when it is in the fringe fields.Thus, although an ion would be stable within the electrodestructureproper, it may not be received in the electrode structureproper due to its unstable trajectory in the fringe fields. This greatlyreduces the transmission of ions of a given charge-to-m ass ratio due totheir rejection while within the fringe fields.

U.S. Pat. No. 3,129,327 further teaches that the ion trajectories can bestabilized on passage through the fringe fields provided that the ratioof the dc voltage (U) to the ac voltage amplitude (V) is reduced to alower value than appropriate for use within the elec- SUMMARY OF THEINVENTION The present invention relates to spatial separation offieldsemanating from electrodes, wherein such fields are produced fromsuperpositions of the dc and timevarying voltages placed on theelectrodes. More particularly, the specific application involved hereinrelates to fringe fields produced in the vicinity of ends of theelectrode structure of a quadrupole mass filter.

Although the specific apparatus to which the methods and structuretaught herein are applied is a quadrupole mass filter, the invention canbe utilized in other devices, such as pulsed power devices, wherein fromelectrodes which carry both do and time-varying voltages it is desirableto have fields which are separated in space.

For an understanding of the invention, reference is made to thefundamental equations describing electromagnetic fields known asMaxwells equations. Using these equations for electric fields, it may bederived, for the electric field, EE, within a medium having a dielectricconstant, e, a magnetic permeability, 1.1., and anelectricaLconductivity, 0', that:

V E (ye/c ([8 E/8t [41r0/e] [SE/8H) 2. where c is the velocity of light(3 X 10" cm/sec).

Any time-varying field-can be described in terms of a Fourier integralor series representation, in which a component of this representationwill have a form of:

I? Ee U 3. where E, describes the spatial part of the field and thetemporal part is given by e w', where r is the time, and m 21rf, where fis the frequency in Hertz, and i is the square root of l.

By substitution of equation (3) in equation (2), the following results:

From the foregoing, it will be understood that for materials'andfrequencies wherein 4rro'lwe is very much less than unity, the secondterm in the parenthesis in equation (4) can be neglected in comparisonto the first term in parenthesis and, accordingly, the materialeffectively acts as a dielectric material. If the same quantity,41r0/we, is very much greater than unity, the material acts as aconductor. Thus, at sufficiently high frequencies, the material acts asa dielectric material whereas the same material acts as a conductor atlower frequencies, including the dc case, with a frequency of zero. 7

It is well known from the theory of electromagnetism (see: J. D.JACKSON, CLASSICAL ELECTRODY- NAMICS, 1962, John Wiley & Sons, or otherstandard texts) that at the interface between a vacuum and a dielectricmaterial there must be mathematical continuity of thevector component ofthe electrical field which is tangential to the surface of theinterface, and also mathematical continuity of the product of the vectorcomponent of the electric field normal to the interface and thedielectric constant of the medium. This implies that there is a slightchange of direction of an electric field in going from vacuum into amaterial with a dielectric constant greater than unity and a furtherslight change in direction in emerging from the material into vacuum onthe other side of the material. Such changes of direction areillustrated in CLASSICAL ELECTRO- DYNAMICS, Supra, FIG. 4.7, page 113.There is also a reduction of the strength of the field which occursinside the dielectric material and a corresponding strengthening of thefield in the vacuum adjacent to the dielectric material. For purposes ofthe present inven' tion, the important point is that the presence of apiece of dielectric material in a region otherwise occupied by vacuumcauses only a minimal change in the shape and field strength relative towhat would exist had the dielectric material not been placed there. Itthus follows that materials, which have the proper ratio of conductivityto dielectric constant for fields of a given frequency wherein thematerial functions as a dielectric only slightly distort the fieldsrelative to that which would be present if such material were notpresent.

It is known from the theory of electromagnetism that when conductors arepresent, electric field lines must terminate at the surfaces ofconductors with the direction of the field being perpendicular to theinterface between the conducting material and the vacuum. It followsthat a hollow tube of conducting material placed in an electric fieldshields against the electric field, provides a field free space withinthe tube and at the same time substantially distorts fields outside andadjacent to the tube. Thus, as utilized in the present invention,placement of a tube or similar shape of material which conductselectricity substantially distorts the fields and leave no fields withinthe tube itself. Accordingly, it follows that a tube of material whichacts as a dielectric to the ac fields but as a conductor to the dcfields of a quadrupole mass filter minimally distorts the ac fields andthe ac fields are thus received without substantial distortion withinthe interior of the tube, whereas the tube completely excludes dc fieldsfrom its interior. Thus, a tube or other configuration of a materialwith the proper dielectric constant and electrical conductivity achievesa spatial separation of the ac and dc fringe fields which wouldotherwise exist coincidentin space if the tube were not present.

Typical quadrupole mass filters have ac voltages at frequencies of about10 Hz (w of the order of 10 sec superposed on dc fields. According tothe theory, materials with dielectric constants of the order of tenshould act effectively as dielectrics if their conductivities aresubstantially less than about l electrostatic units. In moreconventional notation, materials act as dielectrics if theirresistivities are very much higher than about 10 ohm-cm. Such materialsare considered to be leaky dielectrics, and are available in knownceramics and ferrite materials.

BRIEF DESCRIPTION OF THE DRAWINGS The invention is illustrated inpreferred embodiments applied to quadrupole mass filters, in which:

FIG. la diagrammatically illustrates the effect of a pure dielectrichaving a tube form inserted into a mass filter;

FIG. lb is similar to FIG. 1a except that thetube is composed of anelectrically conductive material;

FIG. is similar to FIGS. 1a and 1b except that the material acts as adielectric to electric fields generated by the ac voltages and as aconductor to the electric field from a dc voltage;

FIG. 2 diagrammatically illustrates an embodiment wherein a tube inaccordance with the invention is located adjacent to but outside of theelectrode structure of quadrupole mass filter;

FIG. 3 is similar to FIG. 2 except that a funnel or conical member takestheplace of the cylindrical tube;

FIG. 4 is a perspective view of a further embodiment of the inventionwherein the material for effecting separation of the dc and ac generatedelectric fields is in the form of an annulus located at the end of theelectrode structure of a quadrupole mass filter; and

FIG. 5 is a view of an embodiment of the invention similar to Figureexcept that the annulus is replaced by a plurality of separated membershaving dielectric characteristics in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to- FIGS. la, lb,and 1c, FIG. 1a illustrates the effect of having a pure dielectric, inthe form ofa tube 17 extending into a quadrupole filter between theelectrodes at one end thereof, and the change of the electric fieldlines thereby created. Thus, FIG. Ia shows two electrodes 11 and 12 ofpositive and negative polarities, respectively, which are adjacent toeach other. The dot-dash lines designated 14, show the fields line thatwould be present if the tube 17 of dielectric material (shown insection) were not in place. The solid lines designated 15, show thefield lines as they are altered by placing tube 17 of pure dielectricmaterial mounted on electrode 18 (shown in section) in the positionshown. In practice, the field lines 14 and 15 are practically coincidentexcept for slight displacement through tube 17 which is exaggerated inthe Figure. Thus, it is to be understood that the lines 14 and 15 areshown separated for clarity. The principal point of this figure is toillustrate that the tube of dielectric material distorts the field linesonly slightly from the positions they would have in the absence of tube17 of dielectric material. I

' FIG. lb illustrates the situation where a tube 21 of an electricallyconducting material is substituted for tube 17 of dielectric material.The dot-dash lines, designated 19, show the field lines as exist if thetube were absent, and the solid lines 20 illustrate the field lines withconducting tube 21 (shown in section) in place with its end away fromthe mass filter being connected to ground through an electrode 18 (shownin section), upon which the tube 21 may be mounted. This figureillustrates that with tube 21 composed of a conducting material thefield lines are distorted substantially more strongly than with tube 17of dielectric material and that tube 21 excludes fields from theinterior of tube 17. Again, it should be understood that lines 19 and 20have been separated for clarity in FIG. lb. Lines 19 do not pass throughtube 21 and a slight bulge 16 is created'in lines 19 near and towardsthe opening of tube 21 into the electrode structure 11 and 12. Otherwiselines 19 and 20 are practically coincident.

FIG. 10 shows the situation for fringe fields which affect ions receivedat the end entrance of a quadrupole mass filter, the mass filterelectrodes 11 and 12 having superposition of ac and dc voltages placedupon them. The tube 22 mounted on electrode 18 (shown in section) iscomposed of an appropriate material which functions as a dielectric tothe ac electric field lines, shown as dot-dash lines designated 24 andas a conductor to the dc electric field lines shown as solid linesdesignated 25. The effect of tube 22 is to exclude dc fringe fieldswithin the tube 22 and to move the dc fringe fields as compared to theac fringe fields, which continue to extend away from the end of the massfilter and penetrate into the interior region of tube 22. The lines 24and 25 are separated in FIG. 10 as in FIGS. la and lb. However, with theexceptions noted in describing the field lines shown in such figures thefield lines are otherwise practically coincident.

In the configuration shown in FIG. 16, the fact that the material is aconductor to dc electricity ensures that any ions accidentally strikingthe interior walls of tube 22 will have their charge discharged toground through the material itself via electrode 18. Thus, with properselection of materials, the device remains free of charging which wouldafflict any normal dielectric material of very low conductivity.

Materials exist which, in addition to having appropriate values of thedielectric constant and sufficiently high resistivities, have magneticpermeabilities sufficiently high as to act as magnetic shields to anystray magnetic fields which might adversely affect the entry of ionsthroughthe tube and into the mass filter. Known ferrites constitute anexample of such materials.

As presented heretofore, the tube of FIG. 1c is described as a tube ofuniform homogeneous material with the appropriate values for thedielectric constant, magnetic permeability and electrical conductivity.

The invention is not restricted to a geometry shown in FIG. 10. Amodification of this invention, which places a tube 27 mounted onelectrode 18 (shown in section) entirely outside the quadrupole massfilter electrode structure 11 and 12, is illustrated in FIG. 2.

FIG. 3 shows another modification which uses a cone or funnel member 29mounted on electrode 18 (shown in section) of the leaky dielectricmaterial placed so that the point of the cone extends toward or into theelectrode structure 11 and 12 of the quadrupole mass filter.

FIG. 4 illustrates a further modification of the invention as applied tothe ion entrance of a quadrupole mass filters, wherein mounted onelectrode 18, an annulus 41 composed of appropriate material is placedat the entrance end of the electrode structure of the mass filter withthe interior surfaces of the annulus being shaped to improve admissionof ions through annulus 41 into the mass filter electrode structure 11and 12.

FIG. 5 shows still another modification of the invention, applied toquadrupole mass filters, wherein a single piece of leaky dielectricmaterial is replaced by two or more (shown as four) separate pieces 42of appropriate material, the purpose being to prevent currents whichwould be generated by the fields in a solid piece from producing toomuch energy dissipation therein, there being no restriction on thespecific shapes of the separate pieces of leaky dielectric material aslong as the desired function is obtained.

Although FIG. 5 shows the leaky dielectric material physically separatedfrom the ends of the poles 11 and 12 of the quadrupole mass filter, sucha physical separation is not considered necessary provided that theresistivity of the material is sufficiently high, that is, in excess ofabout 10' ohm-cm for a homogeneous material.

Referring again to the FIG. 10 embodiment, an Extranuclear Laboratories,Inc. Mass Filter, Model 162-8, which has as its main electrode (pole)structure a set of four parallel rods 11 and 12 of circular crosssection, measuring about inch in diameter located so that theirlongitudinal axes lie equidistantly on a circle of approximately 1 H32inch in radius which is perpendicular to said longitudinal axes andhaving a length of 8 inches was used. The tube 22 was composed ofmaterial known as 11/32 Ceramag C/l2, (a carbonnickel-zinc ceramic)manufactured by Stackpole Carbon Co., of St. Marys, Pa. This material,which has a dielectric constant of 10 and a volume resistivity of 3.0 X10 ohm-cm, was in the form of a cylindrical tube of /4 inch outsidediameter and /8 inch inside diameter. Tube 22 was mounted in a stainlesssteel end plate for the mass filter case, said end plate constitutingthe grounded electrode 18. The distance of end plate 18 from the ends ofpoles 11 and 12 was approximately 0.20 inch. The end of tube 22 (notconnected to end plate 18) extended a distance of approximately 0.10inch into the electrode structure 11 and 12 of the quadrupole along itsaxis, as shown in FIG. 10. The particular material used had a statedmagnetic permeability of 35. Thus, it provided some shielding of straymagnetic fields from the electric currents used in an ionizer includedin the instrument, which produce ions by electron impact ionization ofgases, located just outside (and away from the rod structure 11 and 12)of the mass filter. With the arrangement described, a very substantialimprovement in the performance of the quadrupole mass filter wasobtained. At masses of about 200 atomic mass units (AMU) (Mercury ions),the mass filter showed an improvement in resolution going from aboutv1,500 FWHM (Full-width-halfmaximum) without the tube to about 5,000 FWHMwithin the tube. The sensitivity of the instrument at mass 200 wasincreased by a factor of about 100. Without the tube of Ceramag C/l2,the minumum kinetic energy of the ions which upon injection to the massfilter produced satisfactory signals was about 9eV; but with the tube inplace, ample signals could be obtained with an ion kinetic energy ofonly about 3.0 eV. Thus, the presence of the tube at the entrance end ofthe mass filter enhanced transmission, resolution and reduced the minimum ion energy required to use the mass filter. At greater masses, suchas at the doublet in the spec trum of perfiuorotributylamine, at 614 and615 atomic mass units, whereas the mass filter used without the Ceramagtube installed, was barely capable of fully separating the two masspeaks, with the tube in place, the peaks were fully separated with theratio of the distance between the peaks divided by the width of thepeaks at their base being approximately 5, i.e., a resolution of about6,000 FWHM was achieved, using an ion energy of only about 4 eV. Thisdegree of resolution and sensitivity has never before been achieved inthe mass quadrupole spectrometer technology.

The same general configuration was tried using four differentExtranuclear Laboratories mass filters, and the same improvement inresults were repeated. Using three different tubes of the Ceramagmaterial of the same dimensions also gave reproducibility of results.The experiment was scaled up to use an Extranuclear LaboratoriesMass'Filter, Model 324-9, which has circular rods of inch diameter andlength 9 inches. The longitudinal axes of the rods are locatedequidistantly apart on a circle perpendicular to said axis which has aradius of about 11/16 inch. The Ceramag tube was replaced by a furthertube of the same material with the dimensions approximately doubled. Itwas found that the results were again duplicated, and the resultsfurther indicated the separation of the mass doublet N and CO, bothoccurring at nominal mass 28 AMU, with a fractional difference ofapproximately 1 part in 3,000, and of separation of isotopes of H D, DHe and other species in the first four atomic mass units.

Addition of a similar tube to the exit end of the mass filter furtherenhanced the transmission and resolution characteristics of the massfilters, although it appears that the conditions at the entrance end ofthe mass filter are more critical than those at the exit end of the massfilter.

In a second configuration, namely that of FIG. 2, a mass filter of roddiameter inch was used. However, the Ceramag tube 27 had an outsidediameter of approximately r inch, so that it could not be insertedwithin the poles 11 and 12 of the mass filter electrode structure. Thelarger diameter tube 27 was placed just beyond the ends of the poles 27of the mass filter. The presence of tube 27 gave some improvement in thecharacteristics of the mass filters performance, but the improvementswere not as great as when the smaller tube 22 which inserted into theentrance of poles 11 and 12 shown in FIG. 1c was used.

As shown in FIG. 3, the tube of Ceramag material was replaced by a cone29 of the same material. Cone 29 had a wall thickness of approximately3/32 inch, an aperture opening of approximately /a inch diameter, and acone half-angle of approximately 45. The end of cone 29 wasapproximately coincident with the end of poles 11 and 12 of the massfilter. It was found that cone 29 also improved the performance of themass filter, but, again not to the extent of the tube 27 arrangementshown in FIG. 1c.

On the basis of experience with the arrangement disclosed with referenceto FIGS. 2 and 3, the configuration shown in FIG. 1c is considered thepreferred embodiment. However, it is anticipated that furtherconfigurations, arrangements, and materials may result in furtherimprovements. In particular, the experience gained to date suggests thatmaterials with higher magnetic permeabilities and lower bulkresistivities can be used (such as Stackpole Carbon Companys Ceramagmaterial No. C/9), without deleterious effects from conductive anddielectric heating. Ceramag C/l2 has been used rather than a material oflower resistivity to avoid problems of excessive heating of the materialduring initial tests of the invention.

It will be understood by those skilled in the art from the foregoingdisclosure that the material for separating the electrical fields may beutilized in shapes other than those illustrated and described. Forexample, in lieu of a cylindrical tube 22, a tube having rectangular orsquare cross-sectional configuration may be employed. Also, the shapemay conform, more or less, to the shape of the space at the insertion ofthe tube between the four poles.

Having described my invention, what I claim as new and desire to coverby Letters Patent of the United States is:

1. In a method of mass analysis which utilizes a quadrupole mass filterwhich comprises the steps of producing ions, causing the introduction ofsaid ions into the space between the poles of the quadrupole massfilter, and causing the transmission of only those ionsof a selectedmass-to-charge ratio through the space between said poles, theimprovement comprising the use of a field separation means adjacent atleast one of the end of said poles composed of a material whichfunctions substantially as a high dielectric to ac electric fields andsubstantially as a conductor to substantially dc electric fieldsemanating from said pole, said separation means configured to allow saidtransmission of ions and to shield them during said transmission atleast in part from the substantially dc electric fields.

2. A method in accordance with claim 1, wherein said material which isproviding said shielding during said transmission of the ions has adielectric constant of between about 1 and 50, a magnetic permeabilityof between about 1 and 1,000, and a resistivity in excess of aboutohm-cm but not so high as to be unable effectively to conduct awaycurrent caused by ions striking the material during said transmission ofions.

3. A method in accordance with claim 1 wherein said field separationmeans is located proximate the entrance end of said poles for thereceipt and said shielding of ions transmitted into the mass filter.

4. A method in accordance with claim 1 wherein said field separationmeans is located near the exit end of said poles for said shielding ofions transmitted from the pole structure of the mass filter.

5. In a method for improving the efficiency of injection and/ortransmission of ions passing through quadrupole mass filters whichcomprises the steps of producing ions and transmitting ions into,through and from the region between the poles of the quadrupole massfilter, the improvement comprising the use of field separation meansplaced at at least one end of the mass filter pole structure, said fieldseparation means being composed of material which functionssubstantially as a high dielectric to the ac fields and substantially asa conductor to the substantially dc fields of the mass filter, saidfield separation means configured to permit said transmission of ionsand to shield the ions in said transmission at least in part from saidsubstantially dc fields, the geometries of the arrangement being such asto make the ac fringe fields at a given relative field strength in spaceextend relatively farther away from the ends of the mass filter than thedc fringe fields wherein the relative fringe field strength is definedas the strength of the field at a given point divided by the strength ofthe corresponding field within the mass filter electrode structure. 6. Amethod in accordance with claim 5 wherein said field separation meansproviding said shielding for ions is placed at the entrance of the massfilter pole structure.

7. A device for improving the efficiency of injection and/ortransmission ofions passing through quadrupole mass filters, comprisinga tube of material inserted from at, least one of the ends of the poleelectrode structure of the mass filter along the axis and into the spacebetween the four electrodes of the mass filter whereby ions passingthrough said space transit through said tube, the end of said tubedirected away from the mass filter being electrically connected toground, said material of the tube being such that it functionssubstantially as a high dielectric to the ac fields and substantially asan electrical conductor to the substantially dc fields of the massfilter.

8. A device in accordance with claim 7 wherein said tube is insertedinto the entrance end into said space between the four electrodes of themass filter.

9. A device in accordance with claim 7 wherein said tube issubstantially cylindrical in form.

10. A device in accordance with claim 7 wherein said tube issubstantially in the form of a truncated cone.

11. A device in accordance with claim 7 wherein said material is a leakydielectric which has a dielectric constant of between about 1 and 50, aresistivity in excess of about 10 ohm-cm but not so high as to be unableto conduct away any ion current which may strike the ma-' terial and amagnetic permeability between 1 and 1,000.

12. A device in accordance with claim 11 wherein said material is ahomogeneous ferrite.

13. A device for improving the efficiency of injection and/ortransmission of ions passing through quadrupole mass filters, the devicecomprising field separation means substantially in the shape of a cone.of material with an opening therethrough for the transit of ions passingthrough the mass filter, said cone being characterized in that it actssubstantially as a dielectric to ac fields and substantially aselectrical conductor to dc fields of the mass filter, the smaller end ofsaid field separation means being located near at least one end of saidquadrupole mass filter electrode structure, and a circuit means beingprovided for electrically grounding said field separation means.

14. A device in accordance with claim 13 wherein said material is aleaky dielectric which is characterized by having a dielectric constantof between about 1 and 50, a resistivity in excess of about ohm-cm butnot so high that it is unable effectively to conduct away ion currentsthat might strike the material and a magnetic permeability of betweenabout 1 and 1,000.

15. A device in accordance with claim 14 wherein said end comprises theentrance end into the quadrupole mass filter electrode structure.

16. A device for improving the efficiency of injection and/ortransmission of ions passing through a quadrupole mass filter, saiddevice comprising at four pieces of material which are symmetricallydisposed about the axis of the mass filter adjacent at least one end ofthe quadrupole mass filter electrode structure to receive between theions that travel through the mass filter, said pieces of material beingcharacterized by functioning substantially as a conductor to thesubstantially dc fields and a high dielectric to the ac fields of themass filter whereby said pieces of material comprise means which causethe ac fringe fields of the mass filter to extend relatively fartheraway from at least one end of the mass filter than the dc fringe fields,a grounded circuit connected to said pieces of material to conduct awaycurrent caused by ions striking said pieces of material.

17. A device in accordance with claim 16 wherein said material ishomogeneous.

18. A device in accordance with claim 16 wherein said material is aleaky dielectric which is characterized by having dielectric constant inthe range of about 1 and 50, resistivity in excess of 10 ohm-cm but notso high that it it is unable effectively to conduct away current causedby ions striking the pieces of material and a magnetic permeability inthe range of about 1 and 1,000.

19. A device in accordance with claim 18 wherein said material is aferrite.

* l =l =l

1. In a method of mass analysis which utilizes a quadrupole mass filterwhich comprises the steps of producing ions, causing the introduction ofsaid ions into the space between the poles of the quadrupole massfilter, and causing the transmission of only those ions of a selectedmass-to-charge ratio through the space between said poles, theimprovement comprising the use of a field separation means adjacent atleast one of the end of said poles composed of a material whichfunctions substantially as a high dielectric to ac electric fields andsubstantially as a conductor to substantially dc electric fieldsemanating from said pole, said separation means configured to allow saidtransmission of ions and to shield them during said transmission atleast in part from the substantially dc electric fields.
 2. A method inaccordance with claim 1, wherein said material which is providing saidshielding during said transmission of the ions has a dielectric constantof between about 1 and 50, a magnetic permeability of between about 1and 1,000, and a resistivity in excess of about 105 ohm-cm but not sohigh as to be unable effectively to conduct away current caused by ionsstriking the material during said transmission of ions.
 3. A method inaccordance with claim 1 wherein said field separation means is locatedproximate the entrance end of said poles for the receipt and saidshielding of ions transmitted into the mass filter.
 4. A method inaccordance with claim 1 wherein said field separation means is locatednear the exit end of said poles for said shielding of ions transmittedfrom the pole structure of the mass filter.
 5. In a method for improvingthe efficiency of injection and/or transmission of ions passing throughquadrupole mass filters which comprises the steps of producing ions andtransmitting ions into, through and from the region between the poles ofthe quadrupole mass filter, the improvement comprising the use of fieldseparation means placed at at least one end of the mass filter polestructure, said field separation means being composed of material whichfunctions substantially as a high dielectric to the ac fields andsubstantially as a conductor to the substantially dc fields of the massfilter, said field separation means configured to permit saidtransmission of ions and to shield the ions in said transmission atleast in part from said substantially dc fields, the geometries of thearrangement being such as to make the ac fringe fields at a givenrelative field strength in space extend relatively farther away from theends of the mass filter than the dc fringe fields wherein the relativefringe field strength is defined as the strength of the field at a givenpoint divided by the strength of the corresponding field within the massfilter electrode structure.
 6. A method in accordance with claim 5wherein said field separation means providing said shielding for ions isplaced at the entrance of the mass filter pole structure.
 7. A devicefor improving the efficiency of injection and/or transmission of ionspassing through quadrupole mass filters, comprising a tube of materialinserted from at least one of the ends of the pole electrode structureof the mass filter along the axis and into the space between the fourelectrodes of the mass filter whereby ions passing through said spacetransit through said tube, the end of said tube directed away from themass filter being electrically connected to ground, said material of thetube being such that it functions substantially as a high dielectric tothe ac fields and substantially as an electrical conductor to thesubstantially dc fields of the mass filter.
 8. A device in accordancewith claim 7 wherein said tube is inserted into the entrance end intosaid space between the four electrodes of the mass filter.
 9. A devicein accordance with claim 7 wherein said tube is substantiallycylindrical in form.
 10. A device in accordance with claim 7 whereinsaid tube is substantially in the form of a truncated cone.
 11. A devicein accordance with claim 7 wherein said material is a leaky dielectricwhich has a dielectric constant of between about 1 and 50, a resistivityin excess of about 105 ohm-cm but not so high as to be unable to conductaway any ion current which may strike the material and a magneticpermeability between 1 and 1,000.
 12. A device in accordance with claim11 wherein said material is a homogeneous ferrite.
 13. A device forimproving the efficiency of injection and/or transmission of ionspassing through quadrupole mass filters, the device comprising fieldseparation means substantially in the shape of a cone of material withan opening therethrough for the transit of ions passing through the massfilter, said cone being characterized in that it acts substantially as adielectric to ac fields and substantially as electrical conductor to dcfields of the mass filter, the smaller end of said field separationmeans being located near at least one end of said quadrupole mass filterelectrode structure, and a circuit means being provided for electricallygrounding said field separation means.
 14. A device in accordance withclaim 13 wherein said material is a leaky dielectric which ischaracterized by having a dielectric constant of between about 1 and 50,a resistivity in excess of about 105 ohm-cm but not so high that it isunable effectively to conduct away ion currents that might strike thematerial and a magnetic permeability of between about 1 and 1,
 000. 15.A device in accordance with claim 14 wherein said end comprises theentrance end into the quadrupole mass filter electrode structure.
 16. Adevice for improving the efficiency of injection and/or transmission ofions passing through a quadrupole mass filter, said device comprising atfour pieces of material which are symmetrically disposed about the axisof the mass filter adjacent at least one end of the quadrupole massfilter electrode structure to receive between the ions that travelthrough the mass filter, said pieces of material being characterized byfunctioning substantially as a conductor to the substantially dc fieldsand a high dielectric to the ac fields of the mass filter whereby saidpieces of material comprise means which cause the ac fringe fields ofthe mass filter to extend relatively farther away from at least one endof the mass filtEr than the dc fringe fields, a grounded circuitconnected to said pieces of material to conduct away current caused byions striking said pieces of material.
 17. A device in accordance withclaim 16 wherein said material is homogeneous.
 18. A device inaccordance with claim 16 wherein said material is a leaky dielectricwhich is characterized by having dielectric constant in the range ofabout 1 and 50, resistivity in excess of 105 ohm-cm but not so high thatit it is unable effectively to conduct away current caused by ionsstriking the pieces of material and a magnetic permeability in the rangeof about 1 and 1,000.
 19. A device in accordance with claim 18 whereinsaid material is a ferrite.